This invention relates to medical monitoring instruments that use at least one light emitting device to illuminate a patient tissue site and is particularly applicable to photoplethysmographic arrangements having at least one detector that receives light from a plurality of light emitting devices and outputs a multiplexed signal in response thereto.
It is a problem in the field of photoplethysmographic medical monitoring instruments to obtain light detector outputs of sufficient magnitude, quality, and stability to accurately measure the desired physiological characteristics of the subject. The light emitting devices that are used in probes have varying performance characteristics, the absorption characteristics of the tissue vary widely according to probe site and subject, and the presence of external influences such as ambient light can cause significant error components in the resultant measurements. These problems can be compounded in arrangements in which the detector receives light from a plurality of emitters and provides a multiplexed output signal in response thereto.
The above-described problems are addressed and a technical advance achieved in the field by the photoplethysmographic instrument of the present invention.
In one aspect, the invention includes a plurality of light emitting devices, a light detector means for receiving light from the light emitting devices, and an amplifier means having a settable gain for amplifying a multiplexed detection signal indicative of the light received by the light detector means, wherein the amplitude of said multiplexed detection signal is maintainable within a predetermined amplitude range. Preferably, the multiplexed detection signal is a signal output by the detector means.
A monitoring means may be provided to monitor the amplitude of the multiplexed detection signal and to provide a digital control signal to set the gain of the amplifying means. In one approach, the multiplexed detection signal may be converted to a digital signal for processing by a digital processor, wherein an extracted amplitude value may be compared with a references value(s) defining said predetermined amplitude range. In the event the compared value is outside of the predetermined range, the digital processor may be preprogrammed to automatically provide a digital control signal to a switching means comprising the amplifier means, wherein an appropriate gain is applied via a transimpedance amplifier to maintain the detection signal within the predetermined range. By way of example, where the photoplethysmographic instrument is utilized with tissue thicknesses significantly greater/less than a predetermined norm, the applied gain may be automatically adjusted upward/downward by the monitoring means.
By virtue of utilizing an amplifying means with an adjustable gain to amplify a multiplexed detection signal fixed drive means may be advantageously employed to provide fixed drive signals for the light-emitting device(s). For example, different, fixed current signals may be applied to each different one of a plurality of laser diodes. The use of a fixed drive means serves to stabilize the output temperature versus wavelength characteristics of the light-emitting device(s), thereby reducing the potential for error associated with wavelength shift. Additionally, such an approach avoids the need for circuitry typically required to adjust the drive level of light emitting device(s).
Of particular note, in arrangements where a plurality of light sources are sequentially activated (e.g., by a time-division multiplexed (TDM) drive circuit) the gain applied by the amplifying means may be selectively set in corresponding relation to each of the successive portions of the resultant time-division multiplexed (TDM) detection signal. More particularly, in TDM applications, a control signal may be provided to effect the separate setting of a gain level to each sequential portion of the TDM detection signal. As will be appreciated, gain levels may be predetermined in relation to each of the light emitting devices based upon the known light intensity output attributes of the light emitting devices employed.
It should also be noted that in TDM applications, the plurality of light emitting devices may be activated so that each TDM detection signal portion includes a first subportion corresponding with activation of a corresponding one of the light emitting devices and a second subportion corresponding with a precedent or subsequent dark time during which all of the light emitting devices are inactive. In turn, the gain level applied to each given TDM detection signal portion may be applied to both of the first and second subportions thereof.
In additional aspects of the present invention, at least one light emitting device and a detector means (e.g. a photodiode detector) may be employed with an amplifying means that comprises bootstrap amplifier configuration or a balanced input transimpedance amplifier configuration. More particularly, in the bootstrap amplifier configuration, a first amplifier may be employed to maintain a substantially zero bias across the detector means and a second amplifier may be employed to present a substantially zero impedance load to the detector means. In this regard, a transimpedance amplifier and bootstrap amplifier may be interconnected across the detector means. Such an arrangement facilitates rapid gain switching in an amplifying means downstream of the detector means (e.g., in a switched gain circuit comprising the amplifying means as described above). Additionally, the noted bootstrap amplifier configuration may be employed with detector means to facilitate rapid gain switching in a drive means for one or more light emitter(s), wherein narrower drive pulses may be employed. Rapid gain switching is of particular merit in the above-noted TDM applications.
As indicated, a balanced input transimpedance amplifier configuration may also be employed in the amplifying means of the present invention. More particularly, first and second transimpedance amplifiers may be interconnected across a detector means, wherein a common gain level is applied to each of the amplifiers by corresponding first and second switched gain circuits. Again, the gain for each circuit may be set by a digital control signal.
In yet another aspect, an inventive apparatus is provided in a TDM arrangement that employs a signal rotating means. More particularly, the apparatus may comprise a plurality of light-emitting devices which are sequentially driven by a time-division multiplexing drive means and a light detector means for receiving a portion of the light that is transmitted by an illuminated patient site. The signal rotating means is provided to receive a detection signal indicative of the light received by the detection means, wherein the detection signal comprises a plurality of sequential portions corresponding with the sequential activation of the light emitters (e.g., the portions may be sequenced in the same order in which the light emitting devices are activated). The rotating means functions to change the order of the detection signal portions in accordance with a predetermined ordering scheme, and output a reordered signal. The signal rotating means may be advantageously employed to reduce switching noise occasioned by demultiplex/multiplex switching in a low pass filter.
In one embodiment comprising one or more of the above-noted aspects, an inventive apparatus uses a time-division multiplexed fixed drive to activate a plurality of emitters, and a detector with an amplifier having a time-division multiplexed input stage whose gain is digitally set to produce roughly uniform magnitude output signals for all of the input signals received as a result of sequentially activating each of the plurality of light emitting devices. The gain that is set for each given xe2x80x9coptical channelxe2x80x9d (e.g. each different detector output signal portion corresponding with a different emitter and the corresponding signal handling/conditioning/processing applied to such portion) is also maintained for an associated dark time measurement for that optical channel. Such approach ensures that both the ambient light compensation and electronic offsets employed in detection componentry are computed for a selected optical channel at the same gain setting that is used to collect the input data generated for the selected optical channel. This individualized optical channel compensation combined with the common amplifier channel compensation and a fixed light source drive improves the accuracy of the resultant computations.
Numerous additional aspects and advantages of the present invention will become apparent upon consideration of the description that follows.